Design of Yb3+ optical bandwidths by crystallographic modification of disordered calcium niobium gallium laser garnets

Abstract

{Ca□}₃[Nb_1−xGa_x□]₂(Ga_1−yNb_y□)₃O₁₂-type cubic Iad garnets (CNGG) have been grown by the Czochralski method, either unmodified or incorporating different crystalline modifiers (Li⁺ or Mg²⁺), and Yb³⁺ as a laser dopant. From nuclear magnetic resonance, single crystal X-ray and powder neutron diffraction studies it is shown that Li⁺ incorporates exclusively into the 24d tetrahedral site of the host, removing tetrahedral-sited Nb⁵⁺ and filling cationic vacancies in this site. A comparison of low temperature (6 K) Yb³⁺ spectroscopy in crystals with different compositions at the tetrahedral sites and a modeling of energy positions of the ²F_7/2(0) and ²F_5/2(0′) Yb³⁺ levels show that the strongest contribution to the Yb³⁺ optical absorption/emission bandwidth is associated with the electric charge of cations/vacancies occupying the two tetrahedra at the shortest distance (3.12 Å) from the central 24c dodecahedral Yb³⁺. Cationic disorder over the remaining four tetrahedral, octahedral and dodecahedral sites also contributes to the Yb³⁺ bandwidth but to a lesser extent. To obtain the largest Yb³⁺ bandwidth, the two nearest tetrahedra must contain cations/vacancies with electric charges as different as possible. Although the decrease in the concentration of vacancies at tetrahedral sites associated with Li⁺ incorporation induces some reduction of the Yb³⁺ optical bandwidth with regard to the unmodified Yb:CNGG crystal, Li⁺ incorporation along with the use of high purity precursors yields crystals with less coloration, longer Yb³⁺ lifetime, and slightly larger thermal conductivity, which favors laser operation performance.